AFFERENT INPUTS MODULATE THE ACTIVITY OF A RHYTHMIC BURST GENERATOR IN THE RAT DISINHIBITED SPINAL CORD IN VITRO. E. Bracci, M. Beato and A. Nistri. Biophysics Sector and INFM Unit, International School for Advanced Studies (S.I.S.S.A.), 34013 Trieste, Italy.
APStracts 4:0064N, 1997.
ABSTRACT
Application of strychnine and bicuculline to the isolated spinal cord of the neonatal rat induces spontaneous bursting of regular rhythmicity (cycle period about 30 s). This phenomenon is important because it shows that a spinal network, made up by excitatory connections only, generates a very reliable rhythmic pattern. In order to find out how signals from the periphery or higher centres might influence the operation of the rhythmogenic network, the present experiments examined whether synaptic inputs from dorsal root (DR) or ventrolateral (VL) afferent fibres could modulate this spontaneous rhythmicity. This issue was addressed with intracellular recording from motoneurons or extracellular recording from ventral roots after eliciting bursting with strychnine plus bicuculline. Single electrical shocks (0.1 ms; intensity 1-4 times threshold) applied to one DR reset spontaneous bursting without altering its period or duration. Repetitive stimulations at periods ranging from 20 to 2 s entrained bursts on a 1:1 basis. Burst duration was shorter at lower stimulation periods while burst amplitude was unchanged. The lowest stimulation period compatible with burst entrainment depended on stimulus strength. At stimulation periods (2 s entrainment was always lost and spontaneous bursts unexpectedly returned even if electrical pulses still elicited ventral root reflexes. Such spontaneous bursts had similar properties as those recorded in the absence of electrical pulses. Analogous results were obtained with VL stimulations. It is concluded that the spinal rhythmogenic network was highly susceptible to external synaptic inputs which paced burst generation while burst duration was adapted to interstimulus interval. A scheme is provided to explain the modulatory role of synaptic inputs as well as the escape of bursting from fast stimulus entrainment in terms of a rhythmogenic network functionally separated from reflex pathways activated by DR or VL tracts. 

Received 12 November 1996; accepted in final form 5 February 1997.
APS Manuscript Number J893-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 20 February 1997